The Grotthuss Mechanism

Proton transport across lipid membranes is a fundamental aspect of biological energy transduction (metabolism). This function is mediated by a Grotthuss mechanism involving proton hopping along hydrogen-bonded networks embedded in membrane-spanning proteins. Using molecular simulations, we have explored the structural, dynamic, and thermodynamic properties giving rise to long-range proton translocation in hydrogen-bonded networks involving water molecules, or ‘water wires,’ which are emerging as ubiquitous H+-transport devices in biological systems. THE GROTTHUSS MECHANISM Hydrogen-bonded networks possess a very special property: they can mediate the long-range translocation of an excess H+ via chemical exchange of hydrogen nuclei. This process was first imagined by De Grotthuss in 1806 to explain the electrochemical dissociation of water in galvanic cells [1],and was first formulated in the context of biological systems by Nagle and Morowitz in 1978 [2]. The elementary exchange step of the Grotthuss mechanism consists of proton transfer between adj scent hydrogen-bonded groups in the network. The repetition of this step along a suitably-oriented chain results in the net transport of one proton from end to end (see Fig. 1), without the need for a proton-carrying molecule to diffuse throughout the system. This hopping is known as the transport of an ionic defect. In order for a second proton to be translocated in the same direction, the inversion of the chain must first take place, because hopping leaves the chain in the opposite orientation. Because the reorientation of each H-bearing group in the chain creates a defect in the cent inuit y of the hydrogen-bonded chain (HB C), the overall reorient at ion process is described a the translocation of a bonding de~ect. Both of these ‘protonhop’ and subsequent ‘turn’ steps are thus required in the directional transport of protons. In this paper, we summarize recent advances in the of both hop and turn steps of the Grotthuss mechanism in systems at the atomic level. detailed description biologically-relevant